The present invention relates in general to simultaneous signal attenuation measurement systems and, in particular, to reducing undesired cross talk in pulse oximeters and other such systems that identify attenuation characteristics associated with individual signal components.
Signal attenuation measurements generally involve transmitting a signal towards or through a medium under analysis, detecting the signal transmitted through or reflected by the medium and computing a parameter value for the medium based on attenuation of the signal by the medium. In simultaneous signal attenuation measurement systems, multiple signals are simultaneously transmitted (i.e., two or more signals are transmitted during at least one measurement interval) to the medium and detected in order to obtain information regarding the medium.
Such attenuation measurement systems are used in various applications in various industries. For example, in the medical or health care field, optical (i.e., visible spectrum or other wavelength) signals are utilized to monitor the composition of respiratory and anesthetic gases, and to analyze tissue or a blood sample with regard to oxygen saturation, analyte values (e.g., related to certain hemoglobins) or other composition related values.
The case of pulse oximetry is illustrative. Pulse oximeters determine an oxygen saturation level of a patient""s blood, or related analyte values, based on transmission/absorption characteristics of light transmitted through or reflected from the patient""s tissue. In particular, pulse oximeters generally include a probe for attaching to a patient""s appendage such as a finger, earlobe or nasal septum. The probe is used to transmit pulsed optical signals of at least two wavelengths, typically red and infrared, to the patient""s appendage. The transmitted signals are received by a detector that provides an analog electrical output signal representative of the received optical signals. By processing the electrical signal and analyzing signal values for each of the wavelengths at different portions of a patient pulse cycle, information can be obtained regarding blood oxygen saturation.
Such pulse oximeters generally include multiple sources (emitters) and one or more detectors. A modulation mechanism is generally used to allow the contribution of each source to the detector output to be determined. Conventional pulse oximeters generally employ time division multiplexing (TDM) signals. As noted above, the processing of the electrical signals involves separate consideration of the portions of the signal attributable to each of the sources. Such processing generally also involves consideration of a dark current present when neither source is in an xe2x80x9conxe2x80x9d state. In TDM oximeters, the sources are pulsed at different times separated by dark periods. Because the first source xe2x80x9conxe2x80x9d period, the second source xe2x80x9conxe2x80x9d period and dark periods occur at separate times, the associated signal portions can be easily distinguished for processing.
Alternatively, pulse oximeters may employ frequency division multiplexing (FDM) signals. In the case of FDM, each of the sources is pulsed at a different frequency resulting in detector signals that have multiple periodic components. Conventional signal processing components and techniques can be utilized to extract information about the different frequency components.
In order to accurately determine information regarding the subject, it is desirable to minimize noise in the detector signal. Such noise may arise from a variety of sources. For example, one source of noise relates to ambient light incident on the detector. Another source of noise is electronic noise generated by various oximeter components. Many significant sources of noise have a periodic component.
Various attempts to minimize the effects of such noise have been implemented in hardware or software. For example, various filtering techniques have been employed to filter from the detector signal frequency or wavelength components that are not of interest. However, because of the periodic nature of many sources of noise and the broad spectral effects of associated harmonics, the effectiveness of such filtering techniques is limited. In this regard, it is noted that both TDM signals and FDM signals are periodic in nature. Accordingly, it may be difficult for a filter to discriminate between signal components and noise components having a similar period.
Cross talk may also be a significant source of undesirable noise in pulse oximeters. As previously mentioned, pulse oximeters measure the attenuation of various color light signals such as, for example, Red and Infra-Red (IR) wavelength signals that are transmitted through or reflected from a suitable patient tissue site. The different colors of light employed by the pulse oximeter may be referred to as channels (e.g., the Red channel and the IR channel). The attenuation measurements for each channel include a time varying component due to pulsing of the patient""s arterial blood and a static component due to absorption of the light signals by venous blood, tissues and other bodily structures. By employing a ratio of the time varying component of the measured attenuation normalized by the static component of the measured attenuation for each channel, pulse oximeters are insensitive to the absolute signal strength of each color light signal and amplitude measurements of each color light signal transmitted through or reflected from the tissue site can be used to compute the patient""s oxygen saturation (SpO2) level. However, offsets, feed throughs and other cross talks that add signal to any of the measured amplitudes can result in errors in the normalized attenuations, thus resulting in errors in the SpO2 level computed therefrom. For example, system bandwidth limitations and photo-detector tailing smear the signal from the emitter. In this regard, the pulse oximeter system may detect the emitter for a particular color light signal as being on when it is off. This smearing feeds the signal from one emitter into the signal(s) from the other emitter(s) resulting in undesired cross talk from one channel into the other channel(s) of the oximeter. By way of further example, some portion of the emitter drive signal(s) may be capacitively coupled into the detector. Such capacitive cross talk creates an offset in the pulse oximeter system that varies between particular probe cable/detector units.
The present invention is directed to a simultaneous signal attenuation measurement system employing code division multiplexing (CDM). The invention allows for analysis of a multiplexed signal to distinguish between two or more signal components thereof based on codes modulated into the signal components. The CDM codes are nonperiodic thereby facilitating various processing techniques for distinguishing the signals of interest from noise or other interference. Moreover, the invention allows for a variety of hardware and processing options that may reduce costs, simplify system operation and improve accuracy of the attenuation measurements. Further, a reduced cross talk pulse oximetry system and method are provided by the present invention. The reduced cross talk pulse oximetry system and method achieve improved accuracy of pulse oximetry measurements using code division multiplexed modulated drive signal waveforms by utilizing demodulation waveforms that are optimized relative to the attenuated light signal components as output by the detector rather than the modulated drive signal waveforms.
According to one aspect of the present invention, codes are modulated into the transmitted signals of a signal attenuation measurement system. The system includes at least two signal sources (e.g., having different wavelengths) that are pulsed by source drives to a medium under analysis. One or more detectors receive the first and second signal from the medium (e.g., after transmission through or reflection from the medium) and output a composite signal reflecting contributions corresponding to each of the transmitted source signals. The detector signal is thus a multiplexed signal composed of at least two signal components. In accordance with the present invention, the source drives are operated to modulate each of the source signals based on a nonperiodic signal or a code. For example, each drive may pulse a corresponding one of the signal sources between a high output or xe2x80x9conxe2x80x9d state and a low value or xe2x80x9coffxe2x80x9d state. It will be appreciated that, depending on the sources employed, substantial photonic energy may be transmitted in the nominal xe2x80x9coffxe2x80x9d state. Accordingly, in the context of the source signals, a code may be conceptualized as a bit stream of xe2x80x9c0sxe2x80x9d and xe2x80x9c1sxe2x80x9d, where xe2x80x9c0xe2x80x9d corresponds to an off state, xe2x80x9c1xe2x80x9d corresponds to an on state, and the bit length corresponds to a base unit of time that generally reflects the shortest pulse length utilized in driving the sources.
The codes define source signals that have nonperiodic characteristics. That is, due to the codes, there is at least a component of each source signal that is not described by a regularly repeating temporal pattern. As will be understood from the description below, however, the codes themselves may be concatenated in the source signal and a periodic modulating signal may carry the coded signal.
A number of preferred characteristics have been identified for the codes. Among these are:
1. the codes for the different sources are preferably mathematically orthogonal;
2. the numbers of 1s and 0s in a code should be about the same;
3. the distribution of 1s and 0s within a code should be fairly even; and
4. the distribution of transitions between 1s and 0s within a code should be fairly even.
These preferences and some bases therefor are described in detail below. The codes utilized in accordance with the present invention preferably have one or more of these characteristics and, more preferably, have all of the noted characteristics.
According to another aspect of the invention, a detector signal is processed in a signal attenuation measurement system to demultiplex the detector signal and extract component information therefrom based on nonperiodic codes. In particular, the detector signal is first processed to provide a processed signal for demultiplexing and the processed signal is then demultiplexed using at least one coded demultiplexing signal that includes a series of values defining a nonperiodic code. Information is thereby obtained regarding first and second signal components of the detector signal. This information can be utilized in an attenuation analysis to determine an attenuation related parameter of a medium under analysis.
The initial processing of the detector signal may include various processing steps and components depending on the specific application and implementation. For example, where the detector signal is an analog signal, initial processing may involve analog to digital conversion. Preferably, such conversion is implemented using a fast analog to digital converter that digitally samples the detector signal multiple times per source cycle. Such a converter in combination with processing techniques enabled by code division multiplexing allows for improved measurement accuracy and hardware implementation options for certain attenuation measurement applications. The initial processing may further or alternatively include signal filtering to reduce undesired components, signal amplification including, e.g., DC rectification to remove or avoid amplifying DC or low frequency components especially in the case of DC coupled sources, and/or other signal enhancement processing.
Preferably, the demultiplexing process involves the use of a unique demultiplexing signal for each signal component of interest, e.g., corresponding to each signal source. In this regard, the same codes used for modulating the source signals may be used to demodulate the detector signal. However, for mathematical convenience, the demodulating codes may be conceptualized as a series of xe2x88x921s and +1s rather than 0s and 1s as discussed above in relation to the modulating codes. The coded demodulating signal may be filtered to compensate for certain wave shape distortions resulting from bandwidth limitations and non-linearities and/or to reduce response at certain frequencies. In addition, the codes may be pre-computed to reduce processing requirements.
It has been found that simply establishing orthogonality of the demodulation functions or vectors based on the modulating signals of the source drives can result in some degree of cross-talk between the channels or impaired noise rejection. In particular, due to distortions resulting from transmission and processing of the signal between the code generator and the digital processing unit that receives the digitized detector signal, as well as other sources of noise, the received signal may vary relative to the transmitted code. Improved performance can be achieved by establishing the demodulation vectors relative to the received signals rather than the originally generated codes.
Moreover, optimization of the demodulation vectors in this regard can be implemented for specific operating environments such as hardware configurations. That is, code distortion may depend on the specific hardware components employed including, in the case of pulse oximetry, the sources, source drive circuitry, cables, detector, detector output circuitry, the components used for amplification and other signal conditioning, the A/D converter and associated circuitry, as well as non-equipment factors such as the transmission medium and effective optical pathlength (including whether the oximetry probe is attached to a finger, ear lobe, nasal septum, etc.) and other ambient influences. For many pulse oximetry applications, the principal variable in this regard is the identity of the probe that is attached to the patient""s appendage.
The use of an oversampling A/D converter as discussed above allows for substantial processing resolution in xe2x80x9cmatchingxe2x80x9d the demodulation vector to the received code. In particular, an oversampling converter may provide many samples per modulation signal pulse cycle, e.g., 20 or more samples, thereby enabling accurate definition of the demodulation vectors for a particular operating environment.
Thus, in accordance with another aspect of the present invention, a demodulation unit for use in a modulated signal system is provided. The modulated signal system includes: a modulation signal generator for generating at least first and second modulation signals; signal transmitting and processing components for transmitting a signal of interest based on the modulation signals and processing the transmitted signal; and a processor for receiving the resulting processed signal. The demodulation vector unit is operative for providing demodulation vectors for use by the processor in demodulating at least two components of the processed signal, where the demodulation vectors are based on the expected processed signal and reflect demodulation signals that are different than the modulation signals.
In a preferred implementation, the modulating signals reflect at least two mutually orthogonal codes for modulating at least two components of the transmitted signal and the demodulating vectors are defined to be orthogonal with respect to each of the components as received at the processor. The modulated signal system may be a pulse oximeter. An associated method involves receiving a processed modulated signal including at least two components modulated in accordance with input modulation signals and demodulating the processed signal using demodulation vectors based on the expected processed signal, where the demodulation vectors reflect demodulation signals that are different than the input modulating signals.
In accordance with another aspect of the present invention, a process is provided for establishing demodulation vectors for a modulated signal system. The system includes a signal generator for transmitting a signal including first and second components modulated in accordance with first and second modulating codes, signal processing components for processing the transmitted signal and a processor for receiving the resulting processed signal. The process for establishing demodulation vectors involves: operating the system to transmit a first transmitted signal corresponding to the first signal component; receiving a first processed signal corresponding to said first transmitted signal as received at the processor; decoding the first transmitted signal to obtain a first received code; generating a first demodulation vector based on (e.g., orthogonal to) the first received code, and repeating such processing steps with regard to a second transmitted signal corresponding to the second signal component.
In the context of pulse oximetry, this process may be conducted as part of a manufacturing or calibration process. For example, demodulation vectors may be established or updated for a given unit, unit type or model based on a variety of operating environments. Thus, a unit under consideration may be operated, for example, in a variety of equipment configurations involving different probes, different cables and/or other equipment variations. For each such permutation of equipment configuration or associated parameters, appropriate demodulation vectors may be established and stored. During use, the appropriate vectors are applied based on the current equipment configuration or operating parameters/conditions.
In accordance with another aspect of the present invention, a demodulation unit is operated based on a recognized operating environment. The demodulation unit is used in connection with a modulated signal system as described above. The system is operated to transmit a signal including at least one component modulated based on a modulation code. The demodulation unit obtains operating environment information related to the processed signal as received at the processor, performs a comparison to stored demodulation information based on the operating environment information, and selectively provides at least one demodulation vector based on the comparison.
The operating environment may be identified manually or automatically. In this regard, a user can manually identify the operating environment by entering appropriate information into the system, for example, in the context of pulse oximetry, identifying the probe, cable and other equipment and/or the patient appendage under consideration. Alternatively, the operating environment may be identified automatically, for example, in the case of pulse oximetry by operating one or more channels of the oximeter to transmit a signal modulated in accordance with known code or codes, and comparing the received signal to a library of processed modulated signals or demodulation vectors to identify any match. In either case, the identified operating environment may be used to select appropriate demodulation vectors or to lock-out unauthorized or otherwise unsupported equipment configurations, thereby better assuring proper performance and patient safety.
According to one more aspect of the present invention, a method for demodulating first and second attenuated signal components within a composite signal output by at least one detector of a pulse oximeter includes the steps of generating first and second demodulation vectors. In this regard, the first and second attenuated signal components correspond to first and second multiplexed signals (e.g., code division multiplexed signals) that are emitted by first and second optical signal sources of the pulse oximeter, attenuated by a patient tissue site and received by the detector. The first demodulation vector generated is for use in demodulating the first attenuated signal component from the composite signal. In this regard, the first demodulation vector is generated to be substantially orthogonal to the second attenuated signal component of the composite signal output by the detector, rather than orthogonal to second signal as emitted from the second optical signal source. The second demodulation vector generated is for demodulating the second attenuated signal component from the composite signal. In this regard, the second demodulation vector is generated to be substantially orthogonal to the first attenuated signal component of the composite signal, rather than orthogonal to the first signal as emitted from the first optical signal source. The method further comprises the steps of demodulating the composite signal with the first and second demodulation vectors to obtain the magnitude of the first and second attenuated signal components, respectively.
In one embodiment, the first and second demodulation vectors are generated prior to using the pulse oximeter, and the first and second demodulation vectors remain fixed while the pulse oximeter is used to monitor a patient. In another embodiment, the first and second demodulation vectors are initially generated prior to using the pulse oximeter to monitor a patient, and the first and second demodulation vectors are then adjusted dynamically while the pulse oximeter is used. In this regard, the first and second demodulation vectors may be adjusted by applying one or more correction factors that are dynamically computed from information included in at least the first and second attenuated signal components of the composite signal. For example, where the first and second signals are Red and IR signals and, thus, the first and second signal components are Red and IR signal components, the correction factor(s) applied may, for example, be a Red signal to IR signal component cross talk correction factor, an IR signal to Red signal component cross talk correction factor, a Red optical signal source capacitive coupling to Red signal component correction factor, a Red optical signal source capacitive coupling to IR signal component correction factor, an IR optical signal source capacitive coupling to IR signal component correction factor, and/or an IR optic al signal source capacitive coupling to Red signal component correction factor.
The first and second demodulation vectors may, for example, be generated in the following manner. Only the first optical signal source of the pulse oximeter is operated and a first data vector output from the detector when operating only the first optical signal source is recorded. Likewise, only the second optical signal source of the pulse oximeter is operated, and a second data vector output from the detector when operating only the second optical signal source is recorded. In this regard, each source may be operated through multiple cycles of its modulation drive code and the results averaged to obtain the first and second data vectors. A first scalar value corresponding to cross talk from operation of the second optical signal source into the first data vector is computed and a second scalar value corresponding to cross talk from operation of the first optical signal source into the second data vector is computed. The first and second scalar values may be computed by normalizing the first and second data vectors and computing the dot product of the first and second data vectors. A first correction vector having a direction opposite the direction of the first data vector and a magnitude given by the first scalar value is formed, and a second correction vector having a direction opposite the direction of the second data vector and a magnitude given by the second scalar value is also formed. The first demodulation vector is then obtained by subtracting the first correction vector from the first data vector, and the second demodulation vector is then obtained by subtracting the second correction vector from the second data vector.
According to one more aspect of the present invention, a method of correcting for undesired non-orthogonal signal components and interferences in a composite signal output by a detector of multi-channel pulse oximeter includes the step of demodulating the composite signal using matched filters that correspond, respectively, to first and second signal components present in the composite signal to obtain at least first and second uncorrected demodulated signal components corresponding to the first and second signal components. The method also includes the step of demodulating the composite signal using matched filters corresponding, respectively, to first and second interferences present in the composite signal to obtain at least first and second demodulated interferences. In one embodiment, the first signal component is associated with a Red wavelength light signal that has been attenuated by a patient tissue site, the second signal component is associated with an IR wavelength light signal that has been attenuated by the patient tissue site, the first interference comprises capacitive coupling from a Red wavelength optical signal source into the first signal component, and the second interference comprises capacitive coupling from the IR wavelength optical signal source into the first signal component. Having demodulated the composite signal to obtain the demodulated interferences and uncorrected demodulated signal components, a first corrected demodulated signal component corresponding to the first signal component is then obtained by subtracting the second uncorrected demodulated signal component and the first and second demodulated interferences from the first uncorrected demodulated signal component. The method may further include the step of demodulating the composite signal using matched filters corresponding, respectively, to third and fourth interferences present in the composite signal to obtain at least third and fourth demodulated interferences. In one embodiment, the third interference comprises capacitive coupling from a Red wavelength optical signal source into the second signal component, and the fourth interference comprises capacitive coupling from the IR wavelength optical signal source into the second signal component. Having also demodulated the third and fourth interferences, a second corrected demodulated signal component corresponding to the second signal component is then obtained by subtracting the first uncorrected demodulated signal component and the third and fourth demodulated interferences from the second uncorrected demodulated signal component.
According to one more aspect of the present invention, a reduced cross talk pulse oximetry system includes at least first and second optical signal sources, at least one detector, an analog-to-digital converter, and a digital processor. The first and second optical signal sources are operable to transmit first and second optical signals in response to modulated drive signals (e.g., code division multiplexed drive signals). The detector is operable to detect the first and second signals after the first and second signals are attenuated by a patient tissue site. The detector outputs an analog composite signal including at least first and second attenuated signal components corresponding to the attenuated first and second signals. The analog-to-digital converter is operable to convert the analog composite signal output by the detector to a digital composite signal having a plurality of sample instances. The digital processor is operable to demodulate the digital composite signal with a first demodulation vector to obtain a magnitude of the first attenuated signal component and is also operable to demodulate the digital composite signal with a second demodulation vector to obtain a magnitude of the second attenuated signal component. The first and second demodulation vectors are established relative to the first and second signal components in the composite signal rather than the modulation drive codes used to drive the optical signal sources. In this regard, the first demodulation vector is substantially orthogonal to the second attenuated signal component and the second demodulation vector is substantially orthogonal to the first attenuated signal component.
The reduced cross talk pulse oximetry system may further include a demodulation vector unit that is operable to provide the first and second demodulation vectors to the digital processor. In this regard, the demodulation vector unit may select the first and second demodulation vectors based on identification of an operating environment. For example, the demodulation vector unit may be configured to receive manually entered information from a user of the pulse oximetry system, with the manually entered information identifying the operating environment. The demodulation vector unit may also be configured to automatically identify the operating environment by operating at least one of the optical signal sources to transmit a signal modulated in accordance with a known code and comparing an attenuated signal received by the detector with a library of processed modulated signals.
Once selected by the demodulation vector unit, the first and second demodulation vectors may be fixed during demodulation of the composite signal, or the digital processor may further be operable to dynamically adjust the first and second demodulation vectors provided by the demodulation vector unit during operation of the pulse oximetry system. In this regard, the digital processor may dynamically adjust the first and second demodulation vectors by subtracting one or more correction factors from the first and second demodulation vectors. The digital processor may compute the correction factor(s) for each sample instance of the digital composite signal and adjust the demodulation vectors accordingly. In one embodiment, the first and second optical signal sources are Red and IR LEDs, the first and second signals are Red and IR signals, the first and second signal components are Red and IR signal components, and the correction factor(s) is/are a Red signal to IR signal component cross talk correction factor, an IR signal to Red signal component cross talk correction factor, a Red optical signal source capacitive coupling to Red signal component correction factor, a Red optical signal source capacitive coupling to IR signal component correction factor, an IR optical signal source capacitive coupling to IR signal component correction factor, and/or an IR optical signal source capacitive coupling to Red signal component correction factor.
These and other aspects and advantages of the present invention will be apparent upon review of the following Detailed Description when taken in conjunction with the accompanying figures.